1. Field of the Invention
The invention relates to the field of steam generators for nuclear power plants and in particular to apparatus for providing resilient support of flow tubes in the U-bend of the tube bundle in a nuclear power steam generator, thereby eliminating gaps between the tubes and the supports and substantially reducing vibration of the tubes due to the motion of fluids.
2. Description of the Prior Art
In a commercial nuclear power plant, thermal energy from the nuclear reactor is carried by the reactor coolant to a heat exchanger in which the delivered heat converts the feedwater into steam. The steam drives the turbine which converts the thermodynamic power into mechanical power. A mechanical coupling between the turbine and a generator delivers mechanical power to a generator which converts mechanical power into electric power.
In the heat exchanger, the reactor coolant flows through a bundle of flow tubes. The bundle has a straight portion followed by a 180 degree "U-turn" after which there is a second straight portion to the manifold which gathers the reactor coolant for return to the reactor for reheating.
In order to maximize the heat flow from the reactor coolant to the working fluid of the steam generator, a large number of small diameter thin wall tubes are contained in the bundle, and the spacing between the tubes assures good heat transfer. Each flow tube is readily set into vibration by the motions of steam and water. Such vibration, if unrestrained, will lead to the destruction of the tube. Consequently, anti-vibration bars have been introduced in the prior art.
The vibration problem is particularly acute in the U-bend of the flow tubes. The anti-vibration bar of the prior art comprises a bar bent into a V-shaped configuration such that two legs are formed with an angle between. The V-shaped anti-vibration bars are inserted between successive columns of the steam generator flow tubes. The ends of the V-shaped anti-vibration bars are connected to the respective ends of bars inserted into the adjacent gaps between columns of tubes. These anti-vibration bars limit the excursion of vibrations, but since there is at least a clearance gap between the bar and every flow tube, a vibration and rattle occurs as the flow tube is driven by the motion of steam and water around it. The result is a fretting and corrosion of flow tubes which leads to subsequent failure and expensive repair.
U.S. Pat. No. 4,285,396 issued to F. Schwoerer on Aug. 25, 1981 and assigned to Wachter Associates, Inc. discloses spring collar devices positioned at intervals along the tubes which permit lateral movement of the tubes but prevent any relative movement between the tube and its associated spring collar device. Any lateral movement is between the adjacent spring collar devices. With this arrangement fretting and corrosion between the outer surfaces of the tubes and their lateral support members are substantially eliminated. This system is for new nuclear steam generators.
U.S. Pat. No. 4,337,827 issued to F. S. Jabsen on July 6, 1982 and assigned to Babcott & Wilcox Company, discloses a support for a helically coiled fluid heat exchanger. In this heat exchanger flow tubes are bent in helical shapes and adjacent turns of the helix are nested in a plurality of support members. Pairs of tubes are nested between support members against a support strip, a spring is placed over the tubes and a second strip and is pressed on the assembly to the desired spring pressure and affixed. The method of vibration elimination of invention '827 is not applicable to standard nuclear steam generator flow tube geometries.
U.S. Pat. No. 4,747,372 issued to R. M. Wepfer et al on May 31, 1988 and assigned to Westinghouse Electric Corp. discloses an apparatus for minimizing the gaps between anti-vibration bars and the flow tubes in the U-bend section of a nuclear steam generator. Anti-vibration bars are provided between columns of flow tubes in the U-bend region and these bars are contoured on one side to correspond to the exact as built and as bent diameter of the tubes as located within the tube bundle. Such custom fitting of tube anti-vibration bars to actual tube size reduces the gap between anti-vibration bar and tube, but the fitting does not completely eliminate such gaps.
There still exists a need for a better apparatus and method for eliminating the gaps between flow tubes and support bars, for minimizing the vibration of flow tubes and for substantially eliminating the fretting and corrosion of flow tubes due to the action of fluids in a heat exchanger.
Accordingly, one object of the present invention is to provide an apparatus and a method which eliminates gaps between tubes and support bars and thereby precludes a rattling vibration. Another object is to resiliently but firmly support the flow tubes in the correct position in the nuclear steam generator. A further object of the invention is to provide a means of gap elimination and vibration reduction which is convenient to install in both existing and new nuclear power steam generators and is a means which is cost effective.